If he gets distracted (hehehe, not limited to boys!), you might bring in some EF (executive function) supports for his writing. It's not too early to begin using Kidspiration or Inspiration software. If you have a tablet, they have app versions. I started my dd with a free app called Popplet. Any mindmapping software will work. Inspiration/Kidspiration happens to be especially fantastic, because it takes the visual map and converts it to a traditional outline form to write from. This helps kids whose brains are *all over the place* slow down, organize, and see that their thoughts actually ARE able to become something linear and orderly.
Like Texas, we did well with Writing Tales. WT is going to use a form of keyword outlining, much like IEW. Remember, ANY of that you can do with the software to make it more visual. You can type, you can scribe for him, and you can even go BIG, writing on large tablets. My dd had a huge initiation hump, where she had to get everything in her brain, wrapping her brain around it before she could start. It was horrible. So the more support you provide in that process, the better. It's never a problem to provide more support, because as they no longer need it they'll naturally pull away or take it over themselves.
I like the How to Report on Books series for that age. It walks them through narrations but with a bit more structure and with some literary elements as a bonus. The Mrs. Renz book projects were fabulous at that age. They're more creative and they can be appealing to kids who have that spatial, creative, distractible thing going. You might also look for things that require very short bursts of writing, like the Michael Gravois books. You can get them as ebooks, or maybe your library will have them. He has books for country projects, etc., and none will require more than short segments of writing.
It's not what you want to hear, but the things that helped my dd most get over this and get more comfortable writing were time (years, age 13/14), typing (so she could get her thoughts out easily, her handwriting was crunchy), and metronome therapy. The instructions for metronome work are on LC, if you search for heathermomster's posts. Basic idea is that anything done with a metronome is targeting the Executive Function portion of the brain that tends to be delayed in distractible kids. I did metronome work with her, following heathermomster's instructions, adding in digit spans for working memory as she got better at the basic moves. Working memory is what you need to hold all your thoughts, organize them, and get your words out. Working memory tends to be very low in kids with struggles with writing, etc. In fact, ANYTHING you do that works on working memory will be good. You could play Ticket to Ride (a board game) every day and build his working memory, and it would in turn improve his ability to hold his thoughts, organize them, and get them out. Do it and see.